24 JOHN H, NORTHROP 



pothesis is that the virus is formed from a precursor. This assumption 

 does not involve any energy mechanism in addition to that already pres- 

 ent in the host cells and has the further advantage that there is a close 

 experimental analogy in the formation of enzymes (Gratia, 1922 ; Nor- 

 throp, 1938). Darlington (1944) has recently suggested a similar 

 mechanism. Potter and Albaum (1943) consider viruses to be "mis- 

 placed enzymes." 



Viruses have also been considered as obligate parasites. This point of 

 view is ably presented by Rivers (1939) and by Burnet (1945), who 

 consider that a virus is a fragment of a degenerate bacterial cell. This 

 fragment has maintained or developed the power of multiplying in the 

 presence of the host cell. Actually the original source of the virus is the 

 only significant difference between these points of view and that of 

 Bordet, who considered that the virus was originally part of a host cell, 

 rather than of a parasitic bacterial cell. It is quite possible that both 

 ideas are correct since the definition of virus is at present largely (and 

 quite arbitrarily) based on size. Vaccinia, for instance, is similar to bac- 

 teria in many ways, and would not be considered a protein even though 

 its peculiar physiological and pathological properties were not known. 



On the other hand the well-defined crystalline viruses, tobacco mosaic 

 and especially bushy stunt of tomatoes, are typical proteins in many re- 

 spects and would never be considered as related to parasites were it not 

 for the pathological changes which they cause. 



Final decision between the possibilities probably cannot be made 

 until the reaction can be produced in vitro and in the absence of host cells 

 (cf. Seifriz, 1939). 



3. Viruses are formed as a rule only in the presence of cells from 

 related species, but may sometimes increase in the presence of 

 cells of widely separated species. 



4. Viruses are immunologically distinct from the host proteins. 



5. The virus protein produced in the presence of appropriate 

 living cells is usually identical with that used for inoculation 

 but may be different. 



6. Inoculation with two viruses results in production of only one, 

 as a rule. In the case of bacteria (Delbriick and Luria) the virus 

 which is inoculated first is produced and not the second. 



7. No evidence of metabolism has been found. 



8. Viruses are of high molecular weight and contain nucleic acid. 

 Neither of these characteristics, however, suffices to distinguish 

 them from normal proteins. 



